You can file an age discrimination charge with the EEOC. Actually, you must do this as a pre-requisite to any legal action. The EEOC will investigate the charge and attempt to resolve it. In some cases they will sue on your behalf if they believe the claim is strong and they are unable to resolve it.
In all cases, when the EEOC process is over, they will issue a "right to sue" letter, at which time you can pursue legal action. The EEOC process will give you a good idea of whether or not you have a decent case. I should warn you that the EEOC process can take a long time, sometimes over a year.
Read here: http://www.eeoc.gov/laws/types/age.cfm
File a charge here, it cost you nothing to file a charge: http://www.eeoc.gov/employees/charge.cfm
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